Drive shafts for vehicles, ships, etc. are generally composed of a metallic solid rod or hollow pipe having connected to both ends thereof a metallic fitting or joint element. With the recent demand for automobiles of reduced weight, not only replacement of metallic materials of the automobile body with FRP but weight reduction of the structural elements have been attracting attention. In particular, because a torque transmitting shaft is a revolving part, replacement of the material therefor with FRP is expected to make a great contribution to total weight reduction and has aroused considerable attention. That is, an FRP-made drive shaft has a weight reduced by a half to a quarter of that of the conventional steel-made one and its use has been expanding in various automobiles.
In pursuit of comfortableness of ships, use of FRP-made drive shafts has been expected to deviate a resonant frequency from the service range. This is because FRP can make its resonant frequency either high or low while maintaining the torsional strength based on the fact that FRP is superior to metals, e.g., steel and aluminum, in specific strength (strength/density) and specific rigidity (modulus of elasticity/density) and that flexural rigidity or torsional rigidity of FRP can arbitrarily be controlled by changing the angle of orientation of fibers.
FRP drive shafts are generally produced by connecting a separately prepared fitting to both ends of a hollow FRP pipe by any means, for example, with an adhesive. However, connection with an adhesive lacks sufficient adhesive strength for transmission of high torsional torque or undergoes reduction in adhesive strength with time. It is also known to shape the connecting portion into a regular polygon, but such shaping requires much labor in working, resulting in low productivity.
Other various means for transmission of high torsional torque has been proposed. For example, it has been suggested to engage a fitting with an FPF pipe, with both having serrations on the connection portions, or to fit a fitting having serrations on the connection portions into an FRP pipe to bite the inner wall of the FRP pipe as disclosed in JP-A-U-53-9378, JP-A-U-54-97541, JP-A-55-159311, JP-A-54-132039, and JP-B-62-53373 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application"; the term "JP-A" as used herein means an "unexamined published Japanese patent application"; and the term "JP-B" as used herein means an "examined published Japanese patent application").
However, the former means encounters with a difficulty in forming serrations on the inner wall of an FRP pipe. If serrations are formed by mechanical processing, the reinforcing fiber is cut to reduce the strength at the connecting portion, resulting in a failure to transmit a high torsional torque.
The same problem also arises in the latter means. That is, the reinforcing fiber at the connecting portion is cut by the serration of the fitting, resulting in a failure to transmit a high torsional torque.
In an attempt to achieve a reliable connection, it has been proposed to cover the joint with a metallic outer ring for reinforcement, but such diminishes the effect of weight reduction as purposed.